Concrete is noncombustible but conducts heat; because of its mass, however, concrete takes a long time to be heated. Spalling involves the separation of thin plates from concrete surfaces. Many reasons for spalling exist, and most have no relation to fires. The primary reason for spalling of interest to fire investigators involves water vapor pressure in concrete due to fire heat. The likelihood of spalling increases with the water content of concrete and fire intensity. Spalling also depends on the type of concrete and its permeability. Heat spalling tests indicate that spalling is not linked to the use of accelerants, except as it may increase fire temperature. In fact, a liquid accelerant on a floor will keep the floor cool and prevent spalling until all of the accelerant is evaporated. Spalling may then occur 20 minutes later, assuming the fire is not extinguished, but will be caused by high temperature and not by the accelerant. 5 figures